Electrochimica Acta 231 (2017) 216–222

Towards core-shell bifunctional catalyst particles for aqueous metal-airbatteries: NiFe-layered double hydroxide nanoparticle coatings ong-MnO2 microparticles

Andreas Fleglera, Stephan Müssiga,b, Johannes Prieschla, Karl Mandela,b,*,Gerhard Sextla,b

a Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082, Wuerzburg, GermanybDepartment of Chemical Technology of Materials Synthesis, University of Wuerzburg, Roentgenring 11, 97070, Wuerzburg, Germany

A R T I C L E I N F O

Article history:Received 13 October 2016Received in revised form 26 January 2017Accepted 30 January 2017Available online 4 February 2017

Keywords:NiFe LDHlayered double hydroxidesmetal-air batteriesOERbifunctional catalyst

A B S T R A C T

Herein, we investigated the synthesis of a bifunctional catalyst particle system for aqueous metal-airbatteries. To target a system which possesses both, oxygen evolution reaction (OER) and oxygenreduction reaction (ORR) capabilities, g-MnO2 microparticles were combined with NiFe layered doublehydroxides (LDH) to a core-shell system. NiFe-LDH can be optimized in its constituency to yield a verylow onset potential (at 10 mA cm�2) for the oxygen evolution reaction of only 569 mV vs. Hg/HgO. Weinvestigated different coating processes (in-situ precipitation coating and sonochemical assisted coating)in order to create a bifunctional system of LDH shell@g-MnO2 core. It was found that the overall catalyticfunctionality of the bifunctional system strongly depends on the coating process, as this ultimatelydetermines the surface nature and thus the behavior in ORR and OER reactions, respectively, of this core-shell system.

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1. Introduction

The demand for energy storage systems for mobile andstationary applications is rapidly increasing. Currently, the mostwidespread energy storage system for mobile applications likenotebooks or mobile phones is the lithium-ion battery (LIB)technology. However, this technology is already exploited close toits limits regarding the extractable theoretical energy density (387Wh kg�1) [1]. Therefore, due to their much higher theoreticalenergy densities, metal-air batteries come into focus as the batterysystems of the future. For instance, the theoretical energy densityof Li-O2 (aq) is 3582 Wh kg�1 and for the Zn-air system, the valuesare still remarkably high at 1086 Wh kg�1 [1]. The workingprinciple of metal-air batteries is that during discharge, oxygen isreduced to hydroxide ions. This reaction is called the oxygenreduction reaction, ORR. During charging, oxygen is formed. Thisreaction is called the oxygen evolution reaction, OER [1,2].Unfortunately, one of the greatest challenges for this kind ofbatteries is the high overpotential by the air-cathode, i.e., at the so

* Corresponding author.E-mail address: karl-sebastian.mandel@isc.fraunhofer.de (K. Mandel).

http://dx.doi.org/10.1016/j.electacta.2017.01.1790013-4686/© 2017 Elsevier Ltd. All rights reserved.

called gas diffusion electrode “GDE” and which occurs with both,the ORR and the OER. To reduce the overpotentials for the OER andthe ORR, respectively, suitable catalysts are needed. That is whythere is an ongoing search among many different classes ofmaterials to find the best catalyst system. Examples of the mostpromising candidates that were reported so far include noblemetals, transition metal oxides and carbon materials [2–4]. Thehighest catalytic activity for the OER, which is also very importantfor electrochemical oxidation of water, is observed for catalystsincluding Pt, Ru, Ir and Ni [4–6]. Detrimental to the use of thesecatalysts are the high costs for the noble metals and the heattreatment, necessary during synthesis [6].

Herein, we report on a system which is free of any noble metals,fast, simple and cheap to synthesize and which shows a promisingpotential. The key material class we focused on in our work is theclass of the so called layered double hydroxides (LDH). LDHs areanionic clays with positively charged metal hydroxide layers whichcan be intercalated with anions such as carbonate [7]. LDH-basedmaterials are interesting for a broad range of applications as theyfor instance may act as adsorbers for anions, as adsorbers for gasesbut also as flame-retardant agents to name but a few [8–15]. Byusing Ni and Fe as metal cations in the LDH structure, a very activeOER catalyst can be obtained, which was reported recently [16–22].

A. Flegler et al. / Electrochimica Acta 231 (2017) 216–222 217

Ideally, a particle with bifunctional properties is favoured, i.e., aparticle system that possesses both, OER and ORR capabilities. Wetargeted this by a core-shell approach where we used the NiFe-LDHas shell, coated on an ORR active microparticle core. The approachto use bifunctional particles instead of mixing both catalysts has acrucial advantage when it comes to processing the materials. In alater application of the catalysts in a gas diffusion electrode it isdifficult to distribute two different types of particles (withdifferent particle size, morphology, surface chemistry etc.)homogenously in the reactive layer of the electrode. Any specialinhomogeneity bears the danger to yield non-controllablefluctuations in the final catalytic performance of the system. Thus,the device design and production process is much easier if only onecatalyst particle is used, which bears a bifunctional activity.

Initially, we investigated the electrochemical OER activity ofNiFe-LDH particles as function of their structure/composition,which was varied by either deliberately intercalating carbonate orrecrystallizing the structure upon a post-synthesis thermaltreatment. From the results of these initial investigations, thebest NiFe-LDH structure was selected to create a bifunctionalcatalyst particle by combining the OER-active NiFe-LDH withmicron-sized g-MnO2 catalyst particles which are known to beORR active. By evaluating different coating approaches, we found arelation between the coating mechanism and the ORR and OERpotential of this bifunctional system.

2. Experimental

2.1. Chemicals

NiCl2�6H2O and FeCl3�6H2O were purchased from SigmaAldrich, Germany at 99.999 % purity grade, NaOH pellets wereobtained from VWR, Germany and Na2CO3 from abcr GmbH,Germany (purity: 99.5 %). The g-MnO2 microparticles weresynthesized as described in an earlier work from us [23]. Potassiumhydroxide pellets, n-propanol and Nafion solution (5 wt-%solution) were purchased from Sigma Aldrich, Germany. Allchemicals were used as received without further purification.

2.2. Synthesis of NiFe-LDH

For the synthesis of the selected LDH system that containsnickel and iron with a molar ratio of 2:1, 0.43 g (1.8 mmol) nickel(II)chloride hexahydrate (NiCl2.6H2O) and 0.24 g (0.9 mmol) iron(III)chloride hexahydrate (FeCl3�6H2O) were dissolved in 10 mldeionized water (Solution I). A Solution IIa was prepared bydissolving 0.24 g (6 mmol) sodium hydroxide (NaOH) in 40 mldeionized water under stirring. Optionally, 0.19 g (1.8 mmol)sodium carbonate (Na2CO3) was added to Solution II (denoted asSolution IIb). Subsequently, Solution I was added to Solution IIa(respectively Solution IIb) drop by drop with continuous stirring for2 min. After stirring the product for another 3 min, the precipitatewas washed. This was done by centrifuging the dispersion at arotation speed of 5000 rpm for 15 min with a Z513 K, HERMLEcentrifuge. Subsequently, the clear supernatant was decanted andthe sedimented LDH was redispersed for 5 min in 25 ml deionizedwater. This procedure was repeated four times. Eventually, theproduct was redispersed in deionized water. The weight fraction ofthe solid content in the final dispersion was determinedgravimetrically by drying a part of the suspension at 120 �C forat least 18 h.

To carry out recrystallization experiments, the whole procedureas described above to prepare NiFe-LDH with and withoutcarbonate was repeated and the final product was taken andrecrystallized for 114 h at 80 �C in deionized water.

2.3. Synthesis of a g-MnO2-core NiFe-LDH-shell composite particlesystem

g-MnO2 microparticles were coated by two different mecha-nisms with NiFe-LDH as follows:

2.3.1. Sonochemical assisted coating100 mg of g-MnO2microparticles were coated with 10 mg NiFe-

LDH (10 wt% coating) by means of sonochemistry as follows: Theg-MnO2 were dispersed in a NiFe-LDH suspension whichcontained 10 mg solid LDH. After the pH was adjusted to 8-9 with1 M potassium hydroxide, the suspension was exposed toultrasound for 5 minutes in total. This was done with a sonichorn (ultrasound device: Sonics & Materials VCX130) whichexposed the dispersion to ultrasound pulses with a length of 2 s inburst mode every 5 s.

2.3.2. In-situ precipitation-coating500 mg g-MnO2-particles were dispersed in Solution IIa.

Subsequently, Solution I, containing Ni2+ and Fe3+ salts, was addeddrop by drop with stirring.

The samples that were received from the coating-procedures A)and B), respectively, were treated identically afterwards asdescribed in the following: To remove any non-deposited LDH,the reaction product was pressure-filtrated through a filter with apore size of 0.8 mm and a pressure of r = 1 bar. The remainingmicron sized composite core-shell particles (g-MnO2-core-LDH-shell) were washed and filtered again three times with 20 mldeionised water. Eventually, the product was dried until weightconstancy for at least 18 h at 120 �C.

2.4. Electrochemical characterization

The catalytic activity (ORR and OER) of the particles wasanalysed by rotating disc electrode (RDE, pine instruments)measurements. 20 mg of catalyst particles were dispersed in7.96 g H2O, 2 g n-propanol and 40 ml Nafion solution (5 wt-%solution) by ultrasonic (Sonics & Materials VCX130) treatment for 5min. To prepare the working electrode, 20 ml of the catalyst ink wasdropped on a mirror polished glassy carbon RDE tip(diameter: 5 mm) and dried for 15 min at 35� C. Allmeasurements were carried out in 1 M KOH(aq) with a glassycarbon rod as counter electrode and Hg/HgO (1 M KOH(aq)) asreference electrode.

The ORR polarization curves were recorded in O2 saturated 1 MKOH(aq) at a potential range between 0.1 V and �0.6 V vs. Hg/HgOand a scan rate of 5 mV s�1 at rotation rates of 100, 140, 250, 400,900, 1600 and 2500 rpm. The ORR onset potential was calculated ata constant current density of �1 mA cm�2 and a rotation rate of900 rpm. The corresponding OER curves were carried out in Arsaturated 1 M KOH(aq) in a potential range between 0.2 and 0.75 Vvs. Hg/HgO and a scan rate of 5mV s�1 at a rotation rate of900 rpm. The OER onset potential was calculated at a constantcurrent density of 10 mA cm�2.

2.5. Analytical instrumentation

The structure and morphology of the LDH particles werestudied by scanning electron microscopy (SEM, Zeiss Supra 25SEM) at 3 keV (field emission). The zeta potential of the particles asfunction of pH was measured with a Malvern Instruments ZetaSizer Nano. For the zeta potential measurements, the pH wasadjusted by 0.1 M NaOH and 0.1 M HCl. The crystal structure of LDHinvestigated using X-ray diffraction (XRD, PANanalytical943006003002 Empyrean Series). The XRD patterns wererecorded, using Cu Ka radiation (l = 0.15406 nm) in a range

218 A. Flegler et al. / Electrochimica Acta 231 (2017) 216–222

between 5 and 80 2u with a step size of 0.00164 2u and a counttime of 60 s. The specific surface area was calculated by theBrunauer-Emmett-Teller (BET) method from the N2 adsorption/desorption isotherms (Quantachrom Autosorb-3B). The fraction ofcoated NiFe-LDH on g-MnO2 was analysed by X-ray fluorescencespectroscopy (XRF, AXIDOS DY 1495). The distribution of NiFe-LDHon g-MnO2-particles was detected by scanning electron micros-copy (SEM, Zeiss Ultra 55) at 5 keV (field emission) and energydispersive X-ray spectroscopy (EDX, Ametek EDAX SiLi-detector).

3. Results and Discussion

Initially, the effects of intercalation of carbonate into the NiFe-LDH and the influence of post-treatment by temperature to enforceLDH recrystallization were investigated with respect to theirinfluence on the catalytic activity for the oxygen evolution reaction(OER). The core-shell system was omitted at this stage and this pre-selection of the best NiFe-LDH structure was conducted only ontheir pure NiFe-LDH.

3.1. Influence of carbonate and recrystallization on the OER catalyticperformance of NiFe-LDH

From co-precipitation of Ni:Fe at a ratio of 2:1 under theconditions as described in the experimental section, nano-NiFe-LDH particles are obtained � as depicted in the SEM image inFig. 1a. The particle morphology is rather round and no plateletsare visible, although the latter would be the expected form of anideal LDH crystal. The same observation is made for the as-precipitated NiFe-LDH when carbonate is present in excess duringthe synthesis (Fig. 1b). The role of carbonate was investigated as itis well known from literature that carbonate preferentiallyintercalates between the layers of LDH and might, therefore, havea noticeable influence on the catalytic performance of the particlesduring the OER. After recrystallization of the sample shown inFig. 1a at 80 �C for 4 days, it was observed that the shape of theparticles changed from round to platelet-like (Fig. 1c). This

Fig. 1. SEM images of NiFe-LDH particle morphology obtained without (a) and with (b) edays: (c) is the recrystallized sample (a); (d) is the recrystallized sample (b).

observation is in accordance with findings we made earlier anddiscussed in more detail in a previous publication to which werefer for further explanations [24].

The specific surface area increased by a factor of about 10 from�8 m2 g�1 to �83 m2 g�1 after recrystallization. The NiFe-LDHsynthesised with an excess of carbonate did not undergo anysignificant change in shape after recrystallization (Fig. 1d)although the surface area increase here as well, namely fromabout 8 m2 g�1 to �180 m2 g�1.

Fig. 2 shows the X-ray diffractograms of the pure NiFe-LDHparticles. The diffractograms of all samples peak around 11� and23� 2u, which can be related to the rhombohedral LDH structureand which indicate the (003) and (006) crystal planes in the LDH[18]. The presence of excessive carbonate during synthesis doesnot have any influence on the crystallinity of the as-precipitatedsamples. However, samples synthesized with an excess ofcarbonate show a better atomic order after recrystallization at80 �C which is indicated by the occurrence of XRD peaks at around61� and 33� 2u. These peaks can be related to the (110) and (113)crystal planes of LDHs. Detailed discussions on the meaning of thisordering can be found in a work we have previously published [24].

As has been reported in literature, recently, [19] NiFe-LDH showan excellent OER activity. In Fig. 3 the OER polarization curves arepresented and compared to a commercial platinum catalyst(Pt@C). Comparing the synthesized LDH particles, the LDH withoutheat treatment and without addition of carbonate ions shows thelowest OER onset potential (569 mV vs. Hg/HgO). It is veryremarkable that this onset potential is over 100 mV lower than theonset potential of the commercial Pt@C catalyst benchmark. TheLDH structure with intercalated carbonate has an OER onsetpotential which is shifted by about 32 mV towards higherpotential.

As can be seen from Fig. 3, recrystallized LDH in general yields apoorer OER activity. Particularly for NiFe-LDH synthesized inpresence of carbonate, recrystallization yields an OER of about742 mV vs. Hg/HgO which is far worse than the OER onset potentialof the Pt@C benchmark. Apparently, the performance of the LDH isbetter, if the in-between-layer sites are not fully occupied, i.e., it is

xcess carbonate presence during synthesis and after recrystallization at 80� C for 4

Fig. 2. X-ray diffractograms of the samples, depicted in SEM images in Fig. 1. All XRDs are normalized in intensity with respect to the (003) peak.

Fig. 3. OER polarization curves of the samples depicted in Fig.1 and of a commercialPt@C catalyst which is shown as benchmark. Measurements were performed in anAr saturated 1 M KOH(aq) at a rotation rate of 900 rpm.

A. Flegler et al. / Electrochimica Acta 231 (2017) 216–222 219

better to not have carbonate intercalated. This can be explained byassuming the interlayer space to be potentially catalytically active,i.e., the more spots are accessible for oxygen, the better is theperformance of the catalyst. What is remarkable and counter-intuitive, however, is the finding that apparently, a higher SBET doesnot mean a better performance. Comparing the samples synthe-sized without excess of carbonate before and after recrystallizationin their OER performance, it can be seen that the non-recrystal-lized, round-shaped particles perform better than the platelets,although the latter have a tenfold higher SBET. The same trend holdsfor the carbonate rich samples. A potential explanation is that aworse atomic ordering of Ni and Fe in the LDH structure, as itprevails in the as-precipitated samples, yields a better reactivitywith oxygen, i.e., the Ni and Fe atoms are more active if they are notin a completely well-ordered state as this might put them in anenergetic minimum and reduce their activity. Ultimately, thisobservation cannot be explained completely at this stage; however,the key finding is that as-precipitated, carbonate-free NiFe-LDHnanoparticles with a poorly ordered structure are best performingregarding the catalytic activity in an OER. Therefore, this system is

selected to create the g-MnO2-core-NiFe-LDH-shell ORR-OERcomposite particles.

3.2. Coating of g- MnO2 with NiFe-LDH

An effective and simple method to coat nano-LDH on amicroparticle core has been reported by us in the past [14]. Themethod works by adjusting the pH of a dispersion of the micron-sized core-particle and the LDH to be coated onto the micro-particles to a value where the surface charge of one species ispositive and the surface charge of the other species is negative.Ideally, the zeta potentials of the two particle systems are as farapart as possible. Once the pH in the dispersion is set, theelectrostatic attraction of the two materials is supported byexposing the dispersion to ultrasound. Due to that, the materialsare forced together and are, so to say, “electrostatically welded”[14]. Fig. 4 shows the zeta potential as a function of pH for bothmaterials, the g-MnO2 and the NiFe-LDH. It can be seen that belowa pH value of about 10, the NiFe-LDH bears, as expected, a positivezeta potential. The isoelectric point (IEP) of the NiFe-LDH wasfound at a pH of about 10. g-MnO2 has a weakly negative surfacecharge above pH 4. Below pH 4 it is positively charged. The IEP is ata pH of about 4. To achieve an electrostatic attraction for the

Fig. 4. Zeta potential of g-MnO2 and NiFe-LDH as function of pH.

Fig. 6. Zeta potential curves of pure g-MnO2, pure NiFe-LDH, in-situ coating ofg-MnO2 during precipitation of NiFe-LDH and sonochemical assisted coating.

220 A. Flegler et al. / Electrochimica Acta 231 (2017) 216–222

coating between NiFe-LDH and g-MnO2, a pH of 8-9 was adjustedand the system was exposed to ultrasound (see experimentalsection).

Besides the sonochemical assisted coating method, in-situprecipitation was used as an alternative approach to coat NiFe-LDHonto g-MnO2, as described in the experimental section.

Fig. 5a shows the SEM image (inset with higher magnification)of the core-shell particles which were coated via the sonochemicalassisted method. The spherical particles have the urchin-like shapeof pure g-MnO2 particles [23]. EDX mapping results for theseparticles, showing the distribution of manganese and nickel, aredepicted in Figures 5b and c, respectively. Nickel could be detectedon all g-MnO2 particles. The mapping results for iron are notshown as the iron peaks overlap with the energy peaks originatingfrom manganese. In Figure 5d, a SEM image (inset with highermagnification) of the core-shell particles which were coated via in-situ precipitation, is shown. No obvious difference to Fig. 5a can beseen. Also for this system EDX mapping was carried out (Figure 5eand f), yielding the same results as for the sonochemical-basedsystem. It can therefore be concluded that both coating procedures,i.e., the in-situ-precipitation coating and the sonochemical assistedcoating, are applicable to achieve a NiFe-LDH coating on g-MnO2.XRD performed on the LDH-g-MnO2 system (not shown) did notgive any hint that any form of alloy formed, thus, the particlesystem can be considered as g-MnO2-core-Ni-Fe-LDH-shell-system.

To characterize the density of the coating, zeta potentialmeasurements as function of pH were performed with thecomposite system. Fig. 6 depicts the zeta potential graphs of thecoated systems, and, for comparison again (from Fig. 4) the graphsfor pure g-MnO2 and pure NiFe-LDH. It can be seen that the zetapotential versus pH curves lie somewhat in-between the twoextremes of the pure systems. This is an indication for a partial, butnot complete coating, which we discussed in an earlier work inmore detail [25]. An increase in surface occupancy of NiFe-LDH ong-MnO2 results in a decrease of the influence of g-MnO2 on thesurface charge of the composite and thus yields a zeta potentialcurve shift closer to that of pure NiFe-LDH. Consequently, thesurface charge of samples with a higher surface loading is

Fig. 5. SEM images (inset: higher magnification) of the core-shell particles: (a) Sonochemsitu precipitation-coating with EDX mapping for (e) manganese and for (f) nickel.

increasingly dominated by NiFe-LDH and results in a shift of theIEP towards NiFe-LDH. Both coated samples show this shift of theIEP towards higher pH values which is, additionally to the EDXfindings, an indication for a successful coating. The IEP of thesample which was coated by sonochemical assistance is shifted toa pH value of 6.2 whereas the IEP of in-situ precipitation coatedg-MnO2 is shifted to a pH value of 7.8. Hence, at this stage, fromthese findings, it can be assumed that the coating of the lattersample covers more surface of g-MnO2 than the coating usingultrasound.

Finally, the overall catalytic performance of the core-shellcomposite system was determined for ORR and OER and comparedto pure NiFe-LDH, pure g-MnO2, a mixture of the pure particles inthe same ratio as present in the core-shell system, and Pt@C. Theresults are shown in Figure 7. As already known from literature,[16–22] the pure NiFe-LDH particles do not show any catalyticactivity for the ORR (as a current density of �1 mA cm�2 is not

ical assisted coating with EDX mapping for (b) manganese and for (c) nickel. (d) In-

Fig. 7. RDE measurements of the pure materials, a mixture of them, bifunctional catalysts, and a commercial Pt@C catalyst. (a) ORR polarization curves in O2-saturated KOHand (b) OER polarization curves Ar-saturated 1 M KOH at a rotation rate of 900 rpm.

A. Flegler et al. / Electrochimica Acta 231 (2017) 216–222 221

reached, it is not possible to calculate an ORR onset potential).However as already shown, the OER onset potential of 569 mV (vs.Hg/HgO) is very low and thus very attractive. In contrast to that, theg-MnO2 catalyst has an ORR onset potential of �245 mV, which isonly 19 mV lower than the ORR onset potential of Pt@C and thusvery attractive as the system is free of noble-metals and, therefore,much cheaper but at almost the same performance. The OER onsetpotential of g-MnO2, however, is too high to be even detectable inthe potential region relevant to be considered.

The core-shell particles are thus an attempt to combine the bestof both systems.

Fig. 7 shows that the core-shell system synthesized by in-situprecipitation of NiFe-LDH onto g-MnO2 bears a good OERbehaviour with an onset potential of 624 mV which is only55 mV higher than for the pure NiFe-LDH catalysts. Unfortunately,the ORR activity is, although slightly better than for pure NiFe-LDHparticles, relatively poor in comparison to the pure g-MnO2.

The core-shell particle system resulting from the sonochemicalassisted coating procedure exhibits a poor OER activity in the samerange as pure g-MnO2, but an improved ORR activity (onsetpotential of �356 mV) compared to pure NiFe-LDH.

To evaluate whether the co-presence of both particles leads tobetter catalytic properties compared to core-shell particles, theperformance of a mixture of both particles in the same ratio as incase of the core-shell system was measured via the RDE method(Fig. 7). It was found that the mixed system has a similar ORR onsetpotential as the core-shell particle system synthesized via in-situprecipitation. However, the OER activity is much lower comparedto the core-shell particle system. Thus, from these measurements,it can be stated that a mixture of the two systems does not comewith an advantage. However, it should be noted that Dresp et al.[26] found that a 1:1 mixture of the ORR catalyst Fe-N-C with theOER catalyst NiFe-LDH/C leads to both, a good ORR as well as OERperformance. With a 1:1 ratio, however, they used a much higherconcentration of OER particles. As such a ratio could not beachieved with our core-shell systems, the performance reportedcannot be directly compared to our system. The potential draw-back of a “just mixed” catalyst system during processing to a finalbattery system was discussed in the introduction.

Taken together all findings, it can be concluded that the core-shell catalyst system synthesized by in-situ precipitation exhibitsan OER/ORR catalytic behaviour more like NiFe-LDH whereas thesituation is exactly vice versa for the sonochemical assisted coatingprocedure. XRF results revealed that the quantitative coatingfraction for both coating methods is equal. In both cases, theweight fraction of the elements Ni + Fe was 6 wt-% and of MnO2 92wt-%. Although for both coating methods the same ratio betweencore and shell is achieved, the sonochemical assisted coatingshows a catalytic activity more like g-MnO2. This might be

explained by assuming that through the introduced ultrasoundenergy, the NiFe-LDH nanoparticles are pushed more deeply intothe urchin-like structure of g-MnO2 in comparison to the in-situprecipitation coating. In the latter case, the NiFe-LDH particlesrather grow on the outer surface of the g-MnO2 particles andtherefore exhibit a more NiFe-LDH-like surface. It should be notedthat the difference of degree of coating penetration into theg-MnO2 structure cannot be revealed by EDX as the X-rayinformation is obtained not only from the outermost surface buta certain “penetration depth”/“depth of origin of information” hasto be taken into account.

Ultimately, it can be concluded that from these findings, werevealed that it is not only crucial which materials are combined ina core-shell system but also how they are combined. The core-shellformation procedure might have a severe influence on the surfacecharacter of the core-shell system which apparently dominates theoverall character and thus the performance of the system incatalytic reactions. This needs to be kept in mind when creatingcore-shell systems.

4. Conclusion

In this work, it could be ascertained that there is an ideal NiFe-LDH constituency to yield an impressive OER activity. This isachieved by avoiding excess carbonate during the LDH synthesisand using the as-precipitated system which does not possess anywell-ordered crystalline character. A bifunctional system withg-MnO2 core and the LDH as shell could be created by two coatingprocedures. Doing so, it was found that the overall catalyticfunctionality of the bifunctional system can be directed moretowards a superior ORR or OER performance, respectively,depending on the degree of LDH coating density/penetrationonto/into the g-MnO2 core which can be controlled by the type ofcoating procedure.

Thus, as the coating procedure might have a strong influence onthe overall performance of such a bifunctional catalyst system,particular attention to this step should be paid in future.

Acknowledgements

The development of LDH with controlled chemistry andmorphology has received funding within the framework of theproject CO-PILOT from the European Union‘s Horizon 2020research and innovation programme under grant agreement No645993.

KM gratefully acknowledges the DECHEMA Max-Buchner-Forschungsstiftung for supporting him with the Max-Buchner

222 A. Flegler et al. / Electrochimica Acta 231 (2017) 216–222

scholarship for research on using sonochemistry to make novelmaterials.

The authors sincerely thank Manfred Römer for the SEManalysis as well as the EDX mappings.

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